class_docstr: Base class for samplers.
  
  All samplers should subclass `Sampler` and define `__iter__` and `__len__`
  methods.
code: def __len__(self):
      raise NotImplementedError
  
imports: import numpy
label_desc: Use image node_id 2 for calling the Sampler obj's underlying member method code with example usage: obj.__len__() without return types